Apparatus and method for part-load operation of forced-flow steam generators



United States Patent,

Alfred Brunner Winterthur, Switzerland 793,031

Jan. 22, 1969 Oct. 20, 1970 Sulzer Brothers, Ltd. Winterthur, Switzerland a corporation of Switzerland Jan. 24, 1968, July 5, 1968 Switzerland 1,110/68 d o es 6s Inventor Appl. No. Filed Patented Assignee Priority APPARATUS AND METHOD FOR PART-LOAD OPERATION OF FORCED-FLOW STEAM GENERATORS 15 Claims; 7 Drawing Figs.

US. Cl 122/451, 137/625. l9

[5!] 1m; Cl Fggd 5 34.

" 50 Field of Search References Cited UNITED STATES PATENTS 3,046,956 7/l962 Kuljian et a] 122/451 3,064,631 I 1/1962 Schwander l22/451 3,134,368 5/1964 Vincent 122/451 Primary Examiner-Kenneth W. Sprague Atrorneyl(enyon & Kenyon ABSTRACT: The throttling means through which the operating medium is passed into the lengths of tubing is controlled in a manner so that a pulsed flow is caused to flow in periodic sequence to the respective lengths of tubing. The throttling means can be controlled by a rotary slide with a developed face or a cam so that the flow is pulsed. Also, the flow can be recirculated under kinetic energy without the use of a circulating pump.

Patented Oct. 20, 1970 l of 6 Sheet /n ven or ,4L FRED BQU/VNER B 72-10 MES 5 Patented Oct. 20, 1970 Sheet In ventor:

Patented Oct. 20, 1970 3,534,711

Sheet 4 0f 6 lnvemon A ALFRED BFQUNNEQ BY Mun/W Patented Oct. 20, 1970 Sheet L of 6 In ven to h A/. FRED BQUNNEA? (wrro% Patented Oct. 20, 1970 Sheet 6 of 6 5 my 7 ON E M M w W m I r w 2 W m APPARATUS AND METHOD FOR PART-LOAD OPERATION OF FORCED-FLOW STEAM GENERATURS This invention relates to an apparatus and method for part load operation of forced-flow steam generators and, particu' larly, to generators having a combustion chamber lined with lengths of tubing for the flow of operating medium.

Heretofore, steam generators of the forced-flow type have been known to have combustion chambers lined with lengths of tubing through which an operative medium flows in parallel relations. Generally, the operating medium has been metered to the individual lengths or groups of lengths of tubing through throttling devices. For example. in one such manner, the metered flow has been regulated by the throttling devices according to a measured temperature value in the lengths of tubing. However, if-such a steam generator is operated at a very small load, then an unstable behavior is observed. That is, the operative medium becomes distributed, within the groups, nonuniformly among the various lengths of tubing causing individual lengths of tubing to become insufficiently cooled. Moreover, in highly-loaded boilers it is possible, at small loads, even on account of the low speed of flow or because of instability, for film-evaporation to occur. This results in local overheating of the tube material and in rupture of the tubes.

Greatly differing efforts have already been made to eliminate these difficulties. For example, it has been proposed to dispose heating surfaces in parallel with the lengths of tubing forming the lining of the combustion chamber in the weakly-heated zones of the boiler and to shut off these heating surfaces during part-load operation for the purpose of increasing the speed of flow of the operating medium in the remaining combustion chamber zones. Other possibilities have been perceived in the installation of throttle valves, dependent on the load, for lengths of tubing; in the installation of floating elements; and so forth. However, none of these solutions have come into practical use, because they are too expensive to make and/or operate. in the case of high-performance boilers there has come into use the process of circulating the operative medium. in such a process, the operative medium, preferably in load dependent quantity, is by means of a special circulating pump or of a jet apparatus, carried from the end of the combustion chamber tubing back to the beginning of the combustion chamber tubing. in the case of steam generators of medium performance, such a process has been avoided for economic reasons. Also, in subcritieal operation, a considerable danger of cavitation arises in the circulation pumps.

Accordingly, it is an object of the invention to eliminate distribution difficulties and film-evaporation during the startingup and during small-load operation in a steam generator.

It is another object of the invention to recirculate the operative medium without the use of a circulating pump.

It is another object of the invention to operate a steam generator of small to medium performance economically and commercially.

It is another object of the invention to avoid local overheating of the tubes of a steam generator.

it is another object of the invention to uniformly distribute a flow of operating medium through individual tubes or groups of tubes of a steam generator.

Briefly, the invention provides an apparatus for and a method of supplying operative medium to the lengths of tubing of a steam generator at least in the lower part-load region in pulsewise manner. This allows the flow speed in the lengths of tubing to fluctuate between a maximum value and a minimum value in periodic alternation and causes a continuous mass flow to be obtained in the various feed pumps, superheaters and turbines connected with the steam generator.

The apparatus is connected to a plurality of groups or banks of tubing lengths which form the combustion chamber walls of the steam generator and includes a throttling means connected in series to each group of tubing lengths to deliver a form of operating medium thereto and a control means connected to the throttling means for actuating the throttling means in periodic sequence to deliver the operating medium to each group of tubing lengths in periodic intervals of time.

In one embodiment, the throttling means is constructed as a distributor having an interval chamber and a plurality of communicating outlet ports while the control means is constructed as an axially movable rotary slide with a developed face within the chamber of the distributor which serves to cover over all but one, or partially two, of the outlet ports. in this way, as the rotary slide rotates, first one outlet port is opened to the flow of operating medium and then this port and an adjacent outlet port are opened partially to the flow until the first port becomes covered over.

in another embodiment, the throttling means is constructed as a spring and cam actuated valve connected to each group of tubing lengths while the control means is constructed as a mo torized cam for actuating each valve in sequence. in this way, as each valve is opened, a flow of operating medium is allowed to flow into the respective group of tubing lengths.

The method of the invention allows the total flow cross section of the lengths of tubing to be increased without the distribution of the operative medium among the individual lengths becoming unequal or nonuniform. it is therefore possible, even in the case of boilers of relatively low performance in which vertical tubes are welded close together to form the combustion chamber, to connect these tubes in parallel with respect to the operative medium. it is thus possible to do without any down-tubes, which are detrimental in the subcritical operation in the evaporation region.

in order to improve stability in the case of small part-loads, the maximum value of the flow speed is set so as to exceed a fixed predetermined minimum value while, considering any of the lengths of tubing, the interval of time during which a certain value situated between the maximum and minimum value, is exceeded, is varied to correspond with the load. The predetermined minimum value of the flow speed is the value that suffices to suppress film-evaporation, while the determined value of the flow speed represents a means value whose magnitude can be suited to the duration of the pulse.

in order to increase the mean flow of the operative medium in the lengths of tubing, the kinetic energy inherent in the operative medium advancing pulsewise is utilized through recirculation of the operative medium without the use of a circulating pump. This increases the heat-loadibility of the com bastion chamber still further. in accordance with a further form of construction, this problem is solved in that operative medium is conveyed back periodically and alternately from the outlet end of the lengths of tubing to the inlet end of the length of tubing or the group of lengths of tubing concerned when the flow speed has been diminished in the tubing lengths.

During the pulsewise supplying of the lengths of tubing an underpressure occurs each time during the interval of diminishing flow speed at the inlet end of the length of tubing. This underpressure has a tendency to brake the advancing column of operative medium. However, by conveying back the operative medium this underpressure is decreased, and at the same time operative medium is circulated through the length concerned. Along with better cooling, a smoother operation is obtained so that pressure peaks are reduced, and thus hydraulic hammering in the system is avoided.

These and other objects and advantages of the invention will become more apparent from the following detailed description and appended claims taken in conjunction with the accompanying drawings in which:

FIG. It schematically illustrates an apparatus of the invention connected to a plurality of lengths of tubes of a steam generator;

FIG. 2 illustrates a part cross-sectional view of a throttling means and control means of the invention;

MG. 3 schematically illustrates a modified apparatus of the invention;

H6. 4 schematically illustrates a modified apparatus according to the invention;

FIG. 5 illustrates a series of mass-flow diagrams for the apparatus of FIG. 4;

FIG. 6 illustrates another modified apparatus according to the invention; and

FIG. 7 illustrates a view taken on line AB of FIG. 6.

Referring to FIG. 1, a feed pump 1 pumps operative medium in known manner into a throttling means constructed as a distributor 2 with four outlet ports 3. Each outlet port 3 is connected via conduits 8 to one of four groups 4 oflengths of tubing 4 todelivcr the operating medium thereto from. the distributor 2 A control means in the form of a rotatable distributor slide 7 is mounted within the distributor 2 to receive the operative medium axially and to conduct the medium in periodic alternation to the four conduits 8 connected to the outlet ports 3. In addition. a collector is connected in common to the downstream side of the groups 4 of tubing to collect the operating medium and to conduct the same through a superheater 6 to a consumer (not shown).

Referring to FIG. 2, the distributor 2 is constructed of a potlike housing 10 which defines a chamber and which is closed by a cap ll with a Bridgman seal. The housing cap 11 also has a connection inflow port 12 for the introduction of operating medium into the chamber. The four outlet ports 13 are disposed in a radial plane 13 and extend outwardly to the conduits 8. A shaft 14 passes through the opposite end, or bottom, of the housing 10 and carries the distributor slide 7. The face 15 of the slide 7 slants relatively to its axis and, with the cylinder face of the distributor slide 7, forms control edges 16 approximately along two crossing helical lines. An inner ball bearing ring 17 with balls is seated on a shoulder (not shown) of the slide 7 to center the distributor slide 7 in the bore in the housing 10. Further, in order to equalize pressure, the chambers at both sides of the distributor slide 7 are connected with one another by an opening 18 in the slide 7 which extends between the shaft 14 and the ball bearing ring 17.

A V-belt pulley 19 which is driven by a drive motor 20 is mounted on the outer end of the shaft 14 so as to be displaced axially as well as to be rotated with the shaft. Also, a lever 21 which is pivotally mounted on a suitable support is connected to the shaft 14 so that the shaft 14 and the distributor slide 7 can be displaced axially dependent on the load. The range of displacement is such that on the one hand the dot dash mark 22 on the distributor 2 is brought into the plane 13 of the out let ports 3 and that on the other hand the control edges 16 are moved to the right as viewed out of the region of the conduits 8.

When starting-up and when operating under minimal load the distributor slide 7 rotates into its left end position as viewed so that the operative medium is conducted each time substantially to only one, and during change over to only two of the groups 4 of lengths of tubing. As the load increases, the distributor slide 7 becomes shifted to the right by means of the lever 21 so that the interval of time during which a certain tube is supplied with operative medium per period is increased. During maximum load, the distributor slide 7 is in its right-hand end position as viewed uncovering the outlet ports 3 so that the motor 18 can then be stopped.

By increasing the number of groups into which the parallel lengths of tubing are divided, distribution of the operative medium can be improved, and pressure-shocks appearing in the system of tubing can be diminished. Also, if it is desired to improve the distribution at maximum load then the distributor slide 7 will not be pushed completely to the right, but will be made to cover the ports 3 periodically.

Referring to FIG. 3, the distributor slide 7 instead of being made single as above is made double, that is, with two axial planes of symmetry set at right angles to one another. Also, the housing is made to carry double the number of outlet ports 3 in comparison with the single arrangement whereby each two ports set opposite one another are connected with one another by conduits.

This arrangement has the advantage that the operative medium pressures acting on the distributor slide 7 cancel one another out, so that the ball bearing 17 is not loaded by them. In addition, check valves 23 are disposed at the outlet end of the lengths of tubing 4' to prevent a flow-back of operative medium out of the just supplied lengths of tubing into the momentarily unsupplicd lengths.

The rotary speed of the distributor slide 7 is made to lie within a range that is limited at the bottom by the allowable temperature variation in the tubes, and at the top by the massinertia of the operative medium situated in the lengths of tubing. When there is a too rapid a change over, it is of course possible for the column of operative medium not to be set sufficiently quickly into movement. and the rotary speed might come into resonance with one form of oscillation of the operative medium.

In a variant from the illustrated examples of construction. the throttling means 2 and the collector 5 might be interchanged with one another under the superheater might be included in the periodically-fed groups of lengths of tubing. In comparison with these variants, the illustrated forms of construction, however, have the advantage that the throttling means because of the smaller specific volume of the operative medium, can be made smaller, or that the superheater. as a damping volume, can protect the turbine from oscillations of the operative medium.

The distributor 2, or an equivalent periodically controlled throttling means such as a battery of valves controlled by camshafts may have an ordinary operative medium feed valve set ahead of them whose function might also be taken over by having the feed regulator act on the lever 21.

An economizer can also be installed ahead of or beyond the throttling means 2.

Referring to FIG. 4. a feed pump 1 pumps operative medium into a distributor 2', from which run four branch conduits 93a, 93b, 93c and 93d. Each of these branch conduits, by way of a valve 94a, 94b, 94c and 94d respectively, leads to a tubing group 40, 4b, 4c and 4d respectively, which have outlets, by way of check valves 23a, 23b, 23c and 23a respectively, into a common collector 5, from which a conduit 98 conducts the operative medium away. The valves 94a to 94d are each operated through a spring-loaded plunger a to 90d and by a cam 92a to 92d for each of them, which cams are driven in common by a motor 91, so as to be opened and closed alternately. A return flow conduit 83 is connected on the one hand to the collector 5 and on the other hand forked into four branches, which are connected, by way of check valves 84a to 84d, with the inlet end of the groups of tubing 4a to 4d.

With the cam positions shown in FIG. 4, the pump 1 pumps operative medium through the opened valve 940 and the group of tubings 4a into the collector 5. The valve 94d was opened in the last quarter-period, but is now however closed. The operative medium in the group of tubings 4d is, because of its inertia, still in movement, so that operative medium out of the return conduit 83 is conducted back, by way of the check valve 84d, into the inlet end of the group 4d.

The valve 940, which during the last-but-one quarter-period was open, is likewise closed. In the corresponding tubing group 411, the movement of the operative medium has almost died out, and only a little operative medium is still conducted back over the partly-opened check valve 840 into the group 40.

The operative medium is completely at rest in the tubing group 4b. Both check valves 23b and 84b are closed.

Referring to FIG. 5, the chronological pattern of the massflow in the tubing groups 4a to 4d is shown in diagrams (a) to (d) whereby the mass-flow areas enclosed in solid lines represent the mass-flow through the valves 94a to 94d, and the areas delimited by dash-lines show the mass-flow through the corresponding check valves 84a to 84d. The diagrams show that through the arrangement of the return flow conduit 83, with the check valves 84a to 84d, the interval of time in each case, during which there is a flow through a group of tubing, is approximately tripled.

Diagram (e) of FIG. 5 shows a slightly toothed curve which represents the sum of the mass-flows conducted through the valves 94a to 94d; while diagram (f) gives the sum of the massflow conducted back through the check valves 84a to 84d. The hatched areas of diagrams (e) and (f) are approximately equal in size. Thus, in the example of FIG. 4, approximately as much operative medium is carried back as if forwarded by the pump, and the total quantity of operative medium advancing in the groups of lengths of tubing is approximately twice as much as the quantity forwarded by the pump 1.

Referring to FIG. 6, a distributor can be used instead of the cam operated valves above. In this case, a feed pump 1 pumps operative medium through a conduit 31 into a housing 32 of the distributor 30, in which a rotary slide 33 turns and is driven by a shaft 34 (FIG. 7). Four outlet ports 3 (of which only one is shown in FIG. 7) are disposed in the housing 32 in a plane E normal to the axis of the housing 32 and are connected over conduit branches 36a to 36! with the four tubing groups 4a to 4a. In a second plane E normal to the axis (FIG. 7), two return flow connectors 37 are provided in the housing 32 which are connected, by way of a return flow conduit 83 having a check valve 86 therein, with the collector 5. The rotary slide 33 has two axial bores 39 and 40 passing therethrough which serve to balance the pressure of the operative medium at the two faces of the slide. The left-hand face 41 as viewed in FIG. 7 is cut at an angle, so that during tubing of the slide, a control edge 42 is produced having a pendular movement in the axial direction relative to the outlet ports 3. Between the right-hand face 43 as viewed in FIG. 7 of the rotary slide 33 and the control edge 42, the control slide has an undercut 44, through which a second control edge 45 is formed.

The rotary slide 33 is turned uniformly by a motor (not shown) at a speed dependent on the load and can moreover be pushed to the right as viewed in FIG. 7 by a load-dependent contrivance (not shown). The position of the slide shown in FIG. 7 corresponds to the low-load of the steam generator.

With the position shown in FIG. 7, the supply to the illustrated outlet port 3 is shut off. After approximately one-eighth of a turn of the rotary slide 33, the operative medium flows through the conduit 31 into the left-hand part of the housing and, during approximately a quarter-turn, out of the illustrated potlet port 3. Thereafter, this port communicates by way of the undercut 44 with the return flow port 37, so that now operative medium becomes sucked in and circulated by way of the check valve 86.

With an increasing load, the rotary slide 33 becomes pushed toward the right, whereby the interval for direct feed of the tubing groups 40 to 4d becomes increased so that these intervals become overlapped.

The slide 33 is dimensioned so that when under full load and pushed completely to the right, its control edge 42 simultaneously uncovers all outlet ports 3. Depending on the mass-flow desired in the tube groups 4a to 411, it may however be advantageous at full load, to still allow the ports 3 to be covered by the two control edges 42 and 45, so that even at full load operative medium becomes conducted back by way of the check valve 86.

Iclaim:

1. In combination with a plurality of groups of lengths of tubing of a steam generator; throttling means connected in series to each said group of tubing lengths for delivering a flow of operating medium thereto, a feed pump connected to said throttling means to supply the operating medium thereto and control means connected to said throttling means for actuating said throttling means in period sequence to deliver the operating medium to each group of tubing lengths in periodic intervals of time.

2. The combination as set forth in claim 1 wherein said throttling means includes a distributor defining a chamber for receiving operating medium from said feed pump and having at least two outlet ports, each port being connected with a respective one of said groups of tubing lengths, and wherein said control means includes a rotary slide having a developed face in said chamber for selectively overlyingsaid ports.

3. The combination as set forth in claim wherein said rotary slide is axially displaceable and has control edges about said developed face disposed along crossed helical lines and wherein said ports are disposed in one radial plane.

4. The combination as set forth in claim 3 wherein said rotary slide has said control edges duplicated thereon and wherein a plurality of ports are disposed about said chamber. the diametrically opposite ports being connected with each other.

5. The combination as set forth in claim 2 wherein said distributor and rotary slide are connected to a boiler-load giver.

6. The combination as set forth in claim 2 wherein said rotary slide has an undercut extending on one side thereof communicating the opposite sides of said rotary slide within said chamber and which further comprises a return flow branch conduit connected downstream of said groups of tubing lengths and to said chamber of said distributor on an opposite side of said rotary slide from said feed pump whereby said rotary slide periodically and alternately blocks and unblocks the flow of operating medium from said return flow branch conduit to said groups of tubing lengths.

7. The combination as set forth in claim I which further comprises a return flow branch conduit connected downstream of said groups of tubing lengths and upstream of each said groups of tubing lengths for circulating operating medium through said groups of tubing lengths, and means in said conduit upstream of each said group of tubing lengths for controlling the amount of operating medium circulated.

8. The combination as set forth in claim 7 wherein said means are check valves.

9. A method for operating a forced-flow steam generator having a combustion chamber lined with groups of tubing lengths at a part-load which comprises the step of metering operative medium to the tubing lengths at least individually in pulsed manner and in periodic alternation whereby the speed of flow of the operating medium in the lengths of tubing changes between a maximum and minimum value.

10. A method as set forth in claim 9 wherein said minimum value is substantially Zero.

11. A method as set forth in claim 9 wherein at a constant load on the steam generator the chronological pattern of the flow speed apart from phase displacement is equal in each of the tubing lengths.

12. A method as set forth in claim 9 which further comprises the steps of setting the maximum value of the flow speed to exceed a fixed predetermined minimum value and of changing the interval of time during which a certain value between said maximum and minimum values is exceeded in corresponding relation with the load.

13. A method as set forth in claim 9 which further comprises the step of circulating the operative medium from the outlet end of the tubing lengths to the inlet ends upon the occurrence of a diminshed flow speed.

14. A method as set forth in claim 13 wherein the operative medium is circulated until an underpressure prevails at the inlet end of the tubing length with respect to the outlet end of the tubing length.

15. A method as set forth in claim 13 wherein the circulation of the operative medium is controlled dependent on the load. 

